Counterfeiting of currencies, consumer goods, documents and a host of important articles have become a major menace and threat to civilized social life. These arc just unethical advantages derived of otherwise splendid inventions made world over, such as high-resolution color printers, digital reprographics and other hi-tech duplicating machines and methods.
Although eradication of such unethical practices is not easily possible, there could be efficient methods to prevent and check such acts. Till date there have been several attempts to achieve security coding on products and this has been a challenging job for most companies and governmental organizations. Several methods that have been assimilated are holograms, watermarks, microreplicated patterns, special inks, fluorescent pigments, magnetic materials, mixtures of fluorescent materials etc.
Several of these strategies have posed disadvantages in various aspects and proportions, for example, the method of using a diffraction phenomenon such as a hologram has a problem in that the cost is difficult to be reduced, because a mark must be formed on the original. The method, in which the magnetic material is applied, involves a necessity of bringing the original into contact with the magnetic head in a hermetic manner. Therefore, a problem arises in this case that the apparatus structure becomes too complicated. The method using photochromism suffers a problem in that most photochromic compounds are unstable. The image recognition method must use a great quantity of data to recognize all the various original documents to be protected, resulting in heavy apparatus load and difficulty in cutting the cost. Security ink ribbons currently available are either too costly or lose their security features before the inked fabric reaches its visual end of life.
Various methods of providing documents, such as negotiable instruments, with tamper evident systems to prevent alteration have been proposed. Likewise, systems have also been developed to prevent reproduction of documents by photocopying to reduce the incidence of fraud. However, in recent years advanced color copiers, which are readily accessible to the general public, can produce nearly exact duplicates of the original document. It is very difficult for the untrained person to distinguish the original from an illicit reproduction. To prevent passing off of the reproduction as the original, efforts have been made to make the original document incapable of being copied or to incorporate authenticating systems into the document. One known method of producing a paper that is useful for preventing fraud in security documents is to print the paper with a chemical in the form of an invisible image. When a bleach solution or an ink eradicator is applied over the area of the invisible image, a colored image appears to confirm its authenticity. However, this method is often undesirable because the bleach solution is corrosive, the ink eradicator contains toxic solvents, and the solution tends to warp or otherwise permanently damage the document. Another method is to print a document with fluorescent ink, which is colorless under visible light, but becomes discernible when exposed to ultra-violet light. Similarly, fluorescent ink may be printed on a document for automatic identification by an electronic scanner such as that disclosed in an U.S. Pat. No. 4,186,020 to Wachtel. The use of fluorescent ink requires access to a source of ultraviolet light or a document verifying electronic scanner. This prevents quick and easy verification of the document. A further prior method is disclosed in another U.S. Pat. No. 4,183,989 to Tooth in which security paper is provided with a strip, thread or planchette having at least two machine verifiable security features, one of which is a magnetic material and a second of which is a luminescent material, an X-ray absorbent or a metal. Since most security documents are printed paper products with different base weight, color and texture requirements, the availability of various papers having these security features is very limited. This system also has the disadvantage of requiring specialized and expensive verifying machines to examine the documents and determine its authenticity. An U.S. Pat. No. 4,726,608 to Walton discloses the use of an opaque coating over authenticating indicia. The image of the indicia is later made visible by scratching off the opaque coating or by applying a solvent to disperse the coating. This system further has the disadvantage of either requiring solvents or producing unwanted dust. Still another method identifying forgeries is by micro-printing where a message, logo, or name is printed in very small type as a border around the face of the document. Generally, the message cannot be read by the naked eye, and requires some magnification to make it legible. Copiers cannot duplicate the micro-printing with sufficiently high resolution, so that on a copy the words become Just a broken line or blur. However, the tellers at a bank or persons accepting the document generally do not have access to magnifiers and thus, a copy will not be easily noticeable. “Sunburst” or “Rainbow” printing is sometimes used to help defeat color copiers in which the document is printed with one intense color at one side of the document which slowly fades into another color across the width of the paper. Eventually the second color intensifies at the opposite side. This is a visual system that is difficult to duplicate on a copier. If the original printing is not available for comparison, it is extremely difficult for the untrained eye to know whether it is a copy or authentic. Another method is disclosed in an U.S. Pat. No. 4,210,346 to Mowry Jr. et al. in which “VOID” or other warning messages is preprinted in halftone or multitone on the document to camouflage the pre-printing. These pre-printed messages are blended into the surroundings of the document so that the human eye sees them as a uniform printing. The pre-printed “VOID” or warning messages will appear on the illegal copy by many color copiers. However, more sophisticated color copiers have been developed in recent years so that a skillful lawbreaker can now reproduce a security document without the preprinted warning messages appearing on the copies. Another method of reducing the risk of fraud is to incorporate a color forming substance into the substrate of the document. To verify the authenticity of a document, a second color forming substance, which is capable of taking part in a color-forming reaction, is applied to the security document to reveal the hidden images or produce color changes. Examples of this form of detection system are disclosed in the U.S. Pat. No. 4,037,007 to Wood and U.S. Pat. No. 4,360,548 to Skees et al. However, the second color forming substance is not generally available at every location where the security documents are handled or whenever it is needed. Thus, these documents can not be readily tested for their authenticity. The above noted security and authentication systems have not entirely prevented unauthorized or fraudulent reproduction of documents. Accordingly, a need still exists for a simple and efficient system, which is able to accurately authenticate a document and distinguish it from a reproduction.
Photochromism, a useful and simple technique, has been extensively employed in the fields of optical memories and imaging. However, most attempts adopting this phenomenon have confronted with a basic material drawback that the created images are destroyed by photoreactions during the “read” process. The wavelength of light used in the various stages of photoinduced imaging, such as “write”, “read” and “erase”, are usually the same or fall in the region where the different forms of the photochromic molecule absorb. Hence, repetitive “read” process even with low intensity light induces the same reaction as “writing” or “erasing” thus causing destruction of created images. Tamaoki et al. have improved upon this strategy with azobenzene derived photochromic molecules, whose photoisomerization process showed a dependence on the intensity of photolysing light. This makes the system more efficient, as the “write” process requires a high intensity light whereas the “read” process requires only low intensity light thereby protecting the created images. However, the “read” process with these materials required an additional fluorescent film for viewing the photoinduced impressions since the photochromic material by itself is not fluorescent. It would be ideal to have inherently fluorescent systems wherein the fluorescence can be destroyed during the “write” process using light of high intensity. This would permit the “read” process to be carried out with no external aid other than exposure to a low intensity ultraviolet light whereby the image will become clear to the naked eye, without resulting in blurring or erasure of the written image.
Fluorescent labels as a technique for security coding and anti-counterfeit labeling are widely used due to the case and reliability of the method. Although several reports and patents focus on this theme, a need still exists for a simple, cost-effective, efficient, and reliable system which is able to accurately authenticate a document and distinguish it from its spurious version. In most fluorescent labels the authentication procedure needs sophisticated instrumentation to detect and verify the authenticity of the secret imaging or coding. Also the resolution of the image would be limited, since in most cases of imaging using fluorescent materials, these are deposited on a substrate in a required pattern or shape. This method however has a limitation to its resolution and will also be reproducible using several techniques. A methodology using light for writing the image or information provides a high resolution and an additional tamper-proof feature to the label. We report fluorescent labeling materials and methods of imaging them with light resulting in recording of permanent and invisible information which can be easily read using handy ultraviolet lamps. Such labels can bear images with high resolution as it involves light induced writing.
The use of fluorescent labels for security coding and anti-counterfeit labeling is of interest due to the ease and reliability of the method. Although several reports and patents focus on this theme, a need still exists for a simple, cost-effective, efficient, and reliable system which is able to accurately authenticate a document and distinguish it from its spurious version
An earlier patent (U.S. Pat. No. 4,186,020) has described the use of ink containing a fluorescent compound, which is printed in a predetermined pattern. Here the ink is printed on a predetermined pattern on a portion of a label and a protective layer is applied over it. The predetermined pattern is either in the form of a bar code or a readily identifiable design. Such techniques have a drawback that the labels can be easily tampered with since the image is printed onto the label at the final stage.
Another method involving the use of photoluminescent materials for authentication purposes is described in U.S. Pat. No. 6,165,609. This patent describes a method of incorporating a taggant compound in a film forming material. The taggant molecule is generally invisible to the naked eye and on irradiation with UV light, the taggant molecule emits light of a different wavelength. Authentication is then carried out by comparing the emitted wavelength with the wavelength of light know to be emitted by the taggant molecule. The light emitted by the taggant is determined by detector capable of determining the wavelength of light emitted. (U.S. Pat. No. 6,297,508) and compared with the expected wavelength of emission.
A more recent patent (U.S. Pat. No. 6,297,508) has described an improved version of this patent in which more than one taggant compound is used in the label. Thus the fluorescent spectrum emitted by such a label will be complex depending upon the nature of the taggant molecules as well the proportion of the two molecules in the label. Such a mixed taggant label will be more difficult to replicate than labels containing a single taggant material.
A problem with the simple use of fluorescent taggant molecules in labels is that once the counterfeiter is aware of the photoluminescer or the combination of the photoluminescers then reproduction of the labels becomes fairly simple. Also photodetectors are required for authenticating the emitted wavelength from such fluorescent labels.
In view of these limitations, incorporation of additional features into fluorescent labels which would make reproduction of the labels even more difficult is desirable. In this patent we describe the use of fluorescent materials as labels onto which different images can be patterned using light of a specific wavelength and intensity. Thus it would be possible for light of specific intensity and wavelength to make desired markings on the labels using suitable masks thus providing an added security feature compared to the fluorescent labels described in the earlier patents. The fluorescent materials used in these labels have been specially selected such that irradiation of the labels with light of specific wavelengths and intensity can result in loss of fluorescence in the irradiated portions.